integration assessment of visiting vehicleintegration
TRANSCRIPT
Integration Assessment of Visiting VehicleIntegration Assessment of Visiting Vehicle Induced Electrical Charging of The
International Space Station
Presented by Leonard Kramer, Boeing, Houston, Texas.
Leonard Kramer, Boeing Company Thomas W. Kerslake, NASA/Glenn Res. Cntr. Joel T. Galofaro, NASA/Glenn Res. Cntr
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Visiting Vehicle Electrical System Integration into ISS
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Soyuz & ProgressVisiting Vehicles are becoming a regular part of International Space Station operations.
When a visiting vehicle docks with ISS the electrical system grounds become bonded.Since the plasma environment is electrically
ESA/ATVESA/ATV
p yconducting… … and the vehicles have variable
solar array working potentials …… it is necessary to perform an
Dragon… it is necessary to perform an assessment of the electrical charging characteristics during mated operations.
Cygnus
Kramer et al. 11th Spacecraft Charging Technology Conference September 20-24, 2010 | 2
JAXA/HTVOrion
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Spacecraft Charging Assessment Emerges as an ISS Integration Requirement
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The Orion vehicle under development by the United States is the planned crew transport
hi l f ll i th d f S Sh ttlvehicle following the end of Space Shuttle missions in 2011…
…and … the electrical charge on
Orion
the International Space Station affects its operation. There exists:
probability of arcing of anodized coating.remote possibility EVA crew electric shockremote possibility EVA crew electric shock.
Therefore, a formal integration assessment of Orion has emerged as a requirement for safety and mission successrequirement for safety and mission success. Our approach:
Augment existing theoretical model with additional electric current characterization of the Orion Solar Array
Kramer et al. 11th Spacecraft Charging Technology Conference September 20-24, 2010 | 3
characterization of the Orion Solar Array.Perform trade analysis to discern an acceptable generic electric current limitation.
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ISS Charging
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V 0
Solar array electron collection at exposed conductors on positively charged array will drive potential on ISS negative.
V > 0
volts
electrons
Pote
ntia
l/v
V = 0ions
P
electrons
ionsFloating P t ti l
Kramer et al. 11th Spacecraft Charging Technology Conference September 20-24, 2010 | 4
V < 0
Potential
Graphic adapted from a Maxell Tech. presentation (Katz).
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Floating Potential, Plasma State & Boeing Plasma Interaction Model.
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Floating Potential Probe – measures potential of the bonded metal ISS structure relative to the plasma.Plasma Interaction Model (PIM) – is a theoretical calculation based on 3 currentbased on 3 current contributions –
Solar ArraysArray Masts
Langmuir probes are used to measure
yRussian MLI
Boeing LPused to measure
plasma density and electron temperature.
P reduction
Kramer et al. 11th Spacecraft Charging Technology Conference September 20-24, 2010 | 5
November 18, 2009
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Solar Array Wings on Orion Vehicle
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Image credit: esc gsfc nasa govImage credit: esc.gsfc.nasa.gov
20 “gore” segments dividedbetween 2 wings
Kramer et al. 11th Spacecraft Charging Technology Conference September 20-24, 2010 | 6
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One “gore” Segment
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20 “gore” segments dividedbetween 2 wingsbetween 2 wings
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Exposed Conductors Collect Electrons
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Cells are covered with glass butbut…
…with small amounts of exposed conductor…
... available to collect electronscollect electrons when the string is biased positive
l ti t threlative to the plasma.
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Cell Coupon in Plasma Chamber
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Positive and negative terminal are shorted together and biased externally.
Electrons collect onElectrons collect on exposed conductor.
Collected current is measured as bias is varied
Kramer et al. 11th Spacecraft Charging Technology Conference September 20-24, 2010 | 9
Collected current is measured as bias is varied.
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Laboratory Current – Voltage Sweep
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The Baseline design described on previous charts was tested at ionospheric environment conditionionospheric environment condition.
This variant has exposed metal end terminals and plasma state adjusted to higher density conditions.
(n0,T0)=(3.51012m3, 0.25eV)
… the variant is an experimental control to illustrate the ff ti f
(n0,T0)=(1012m3, 0.2eV)
effectiveness of covering the conductors.
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For the purpose of ISS integration it is essential that we relate collected current at chamber n0 and T0 to arbitrary n & T
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Langmuir Probe Provides On-Orbit Physical Model
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The probe is a metal element collecting current from the plasmacollecting current from the plasma.
ISS Langmuir Probe I-V sweeps
Electrons collect on exposed conductor.
This spread results from variation in plasma state along ISS orbit.
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Bias (volts) relative to ISS structure ground.
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Model of Visiting Vehicle in PIM
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We derive a scaling relationship between I-V curve measured at chamber n0 and T0 to the I-V curve at arbitrary n and T.
ProbeI
chamber n0 and T0 to the I V curve at arbitrary n and T.
0 7
Chamber Measured I-V curve
eenv A
M0 0
V1+n T TI(V,n,T) = I (V)× × Vn T 1+T
0.7
0T Scales directly with n…
… has more complicated relationship to T
V
The equation is:our model of current collection.implemented as an additional
relationship to T
Kramer et al. 11th Spacecraft Charging Technology Conference September 20-24, 2010 | 12
B
eVXk T
implemented as an additional current source in PIM.
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ORION Charging of ISS
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Four charging scenarios have been simulated.
Eclipse exit is a worst case.
Single and Dual docked Orion simulations exhibit very benignsimulations exhibit very benign charging characteristics.
The exposed metal variant cellInternational Reference Ionosphere
The exposed metal variant cell coupon with (case D) exhibits severe charging.
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Theoretical Currents to ISS with Baseline Orion
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Orion will contribute no adverse charging.
Nominal No-Orion Case
Currents are negligible compared to ISS solarcompared to ISS solar arrays and other components.
Baseline: Single Docked Orion
Dual dock case (not shown) has similar
Kramer et al. 11th Spacecraft Charging Technology Conference September 20-24, 2010 | 14
Dual dock case (not shown) has similar benign charging contribution.
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Theoretical Currents to ISS: Exposed Metal Variant
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It is interesting to compare No-Orion with the exposed metal variant case.
Nominal No-Orion Case
Currents are negligible compared to solar arrays and other components.
~ 50 mA Orion current displace current from solar arrays & ISS
Orion exposed metal variant.
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current from solar arrays & ISS FP approaches -80volts.
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Trade Study
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Orion model was modified to deliver a simple constant current (a parameter) when illuminated in sunlight -- zero current in eclipse. The parameter was varied among 5 10 and 50 mAvaried among 5, 10 and 50 mA.
Establishes a simple visiting vehicle induced current limitvehicle induced current limit that yields negligible impact to ISS-Visiting Vehicle integrated configuration.
Parametric current at 5 mA produces less than 2 volt excursion from nominal, No ,Orion charging.
5milli-amps appears
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Eclipse Exit
5milli amps appears to provide a satisfactory limit.
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Summary & Conclusion
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We measure the electric current collection characteristic from the Orion solar arra ssolar arrays.
We also develop a theoretical scaling of the current incorporated into the Boeing Plasma Interaction Model.g
We find that currents supplied by the baseline Orion SA design impose less than 2 volt excursion from normal charging conditions -- baseline design imposes benign charging characteristics on ISSimposes benign charging characteristics on ISS.
Trade Study conducted provides objective criterion for a generic visiting vehicle current contribution to ISS:
Limiting combined visiting vehicle contribution to 5 milli-amps is found to mitigate adverse charging effects on ISS.Remaining operational challenge to apportion 5 milli-amps among a fleet of several visiting vehicles
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fleet of several visiting vehicles.